An antenna device in the related art disclosed in Patent Literature 1 includes a metal conductor of a flat-plate shape (base plate) functioning as a ground, another metal conductor of a flat-plate shape (opposing conductor plate) disposed oppositely to the base plate and provided with a power-feed point at an arbitrary position, and a short-circuiting portion electrically connecting the base plate and the opposing conductor plate.
The antenna device as above gives rise to parallel resonance using an electrostatic capacitance generated between the base plate and the opposing conductor plate and inductance of the short-circuiting portion at a frequency corresponding to the electrostatic capacitance and the inductance. An electrostatic capacitance generated between the base plate and the opposing conductor plate is determined by an area of the opposing conductor plate. Hence, by adjusting the area of the opposing conductor plate, a target frequency (operating frequency) for transmission and reception by the antenna device can be set to a desired frequency.
Patent Literature 1 discloses a configuration in which a single slot formed in a C-shape is provided to the opposing conductor plate to be parallel to an outer edge portion of the opposing conductor plate. The opposing conductor plate is thus divided to a region on an inner side of the slot and a region on an outer side of the slot.
The configuration as above can achieve two capacitive effects. One is a capacitive effect by the region of the opposing conductor plate on the outer side of the slot and the other is a capacitive effect by the region of the opposing conduction plate on the inner side of the slot. Consequently, the antenna device has two resonance frequencies corresponding to the respective regions. In order to achieve the capacitive effect by the inner region, the slot provided to the opposing conductor plate in Patent Literature 1 has a length with which the slot substantially forms most of a circumferential portion of the inner region.